It has always been said that writers, by the very nature of their art, play God. This is, perhaps, nowhere more clear than in author Briony Tallis' act of inventing a happy ending for a couple who were denied such an outcome in real life as a result of her youthful indiscretions.
Rather than abiding by reality or historical truth, Briony fashions a new truth from the dust of her own imagination and the rib bones of what could have--what should have--been. But in playing God, Briony is also well aware that to go all the way back in time, before The Fall, would be to pretend her sin never existed and that, for her, is equally impossible.
And so, imaginatively omnipotent, but bound by her own immutable laws, Briony grants the deceased couple redemption--paid for, ironically enough, with their own blood--and gives them what only she can: an eternal life of bliss together in a perpetual paradise where they shall never grow old, never share a cross word, and never again face the scourge of her or anyone else's sin.